The Ultimate Black Mage Resources with Damage Calculator

[SeraviEdalborez]

You can start casting the moment you interrupt. The only real delay in it is if you interrupt by moving, you have to stop moving, but it takes just a slight tap of the movement key to interrupt, so that's a negligible delay. In practice, your reaction speed increases the overall delay, but the same is true for the fireweaving idea because you have to take the time to process whether you got a proc or not. And, as I said, there's no delay with the interruption idea if you don't get a proc since you just keep casting as normal, so even if you're wasting a bit more time on the interrupts, it would be counterbalanced by the times you don't have to interrupt (which exceed the times you have to interrupt).

You actually waste more time by "firewaiting" instead of fireweaving.

When you weave a off-GCD immediately after, the status effects granted by the preceding GCD ("+Astral Fire", "+Firestarter")pop up immediately instead of waiting the (approximately) 0.5s for the firebolt to travel. Now you can say "but Seravi, it's the same amount of time so it doesn't matter". Nope. It's about WHEN you have that time.

When you see the effects early by fireweaving, it gives you (a human and not a goddamn robot) time to see the proc and use it. Because you're waiting an enforced 0.5s anyways by forcing the popup to come early. If you firewait, you need extra time to see it and use it. You do not have reflexes fast enough to, with near-perfect accuracy, make firewaiting better than fireweaving. If you do, you're a robot and should report yourself to the appropriate authorities.